The present invention relates generally to space diversity in radio communication systems. In particular the invention relates to an antenna switching diversity system and radio base stations and radio terminals in such a system.
Antenna switching diversity is a special form of space diversity in radio communication systems. It provides the possibility to switch to another antenna if reception is bad.
The DECT (Digital Enhanced Cordless Telecommunications) system, which is a TDMA-TDD (Time Division Multiple Access-Time Division Duplex) system, provides both downlink and uplink antenna switching diversity by selecting one out of two available antennas in the base station for both transmission and subsequent reception. Since transmission and reception is on the same frequency in DECT, the uplink fading is the same as the downlink fading, which implies that the same antenna is preferred for both links.
The GSM and D-AMPS systems, which are TDMA-FDD (Time Division Multiple Access-Frequency Division Duplex) systems, on the other hand, provide only uplink space diversity, which often results in bad reception quality at the mobile terminal. In these systems it is not possible to use one out of two antennas for both uplink and downlink reception and transmission, respectively, since the base station transmits and receives simultaneously. In fact GSM and D-AMPS base stations typically use two antennas for uplink reception and a third antenna for downlink transmission. However, it would be desirable to provide downlink antenna switching diversity for TDMA-FDD systems, such as the GSM and D-AMPS systems.
Furthermore, WO 97/21287 (ADVANCED MICRO DEVICES) describes a combined TDMA-TDD/FDD system. It would be desirable to provide this system with both uplink and downlink antenna switching diversity.
Accordingly it is an object of the present invention to provide a modified TDMA-FDD radio communication system with downlink antenna switching diversity.
It is also an object of the present invention to provide a TDMA-TDD/FDD radio communication system with both downlink and uplink antenna switching diversity.
Another object of the invention is a base station that is modified to fit into such radio communication systems.
Still another object of the invention is a radio terminal that is modified to fit into a modified TDMA-FDD radio communication system as well as a traditional system.
These objects are achieved in accordance with the enclosed claims.
Briefly, the present invention separates transmission from reception both in frequency and time and in both base station and radio terminal. The base station measures uplink reception quality and determines and uses the best antenna for reception. The radio terminal determines reception quality downlink and reports this quality to the base station, which switches transmission antenna for the next transmission if the downlink reception quality is low and maintains the present antenna if it is high. Thus uplink and downlink antenna switching is determined independently for the uplink and downlink. Such a modified system will increase reception quality downlink.